1. Field of the Invention
The present invention relates to motor control in an electric power steering apparatus and the like, particularly to boosting control of power supply voltage supplied to a motor.
2. Description of the Related Art
The electric power steering apparatus imparts steering assist force by an electric motor when a driver steers a steering wheel of a vehicle. FIG. 11 is a block diagram showing an example of the electric power steering apparatus. In FIG. 11, the numeral 50 designates a steering wheel (hereinafter referred to as “handle”) provided in a driver's seat of the vehicle, the numeral 51 designates a shaft whose one end is coupled to the handle 50, the numeral 52 designates a torque sensor which detects a torque applied to the shaft 51 during the steering, and the numeral 53 designates a gear mechanism coupled to the other end of the shaft 51. The numeral 54 designates an electric motor which generates the steering assist force, and the numeral 55 designates a transmission mechanism which transmits the steering force of the handle 50 and the steering assist force of the motor 54 to wheels 56 through the gear mechanism 53. The numeral 57 designates a current detector which detects current being passed through the motor 54, and the numeral 58 designates a speed sensor which detects running speed of the vehicle. The numeral 59 designates a battery which supplies power supply voltage for driving the motor 54, the numeral 60 designates a boosting circuit which boosts the voltage of the battery 59, and the numeral 61 designates a controller including an electronic control unit (ECU) which controls the motor 54.
When the handle 50 is steered, the torque sensor 52 detects the torque which is generated in the shaft 51 by the steering, and the controller 61 computes a target value of the current to be passed through the motor 54 based on the torque value and speed detected by the speed sensor 58. A motor drive circuit (not shown) incorporated in the controller 61 supplies the current to the motor 54 based on the current target value, and the motor 54 is rotated to generate the steering assist force. The current of the motor 54 is detected by the current detector 57, and the detected current value is inputted to the controller 61. The controller 61 compares the detected current value and the current target value, and the controller 61 computes a command value based on a deviation between the detected current value and the current target value. For example, in a case where the motor drive circuit includes a switching element turned on and off by a PWM (Pulse Width Modulation) signal and the voltage is applied to the motor according to a duty ratio of the PWM signal, the duty ratio becomes the command value. The controller 61 performed feedback control based on the command value such that the current having the target value is passed through the motor 54.
For example, Japanese Examined Patent Publication No. H6-51474 described the electric power steering apparatus.
In the electric power steering apparatus, assuming that the voltage of the battery 59 is set at 12 volts, because the current of tens amperes is passed through the battery 59 in the maximum load (running at an extremely low speed or the like) of the steering assist by the motor 54, the motor driving power supply voltage imparted to the controller 61 is decreased due to voltage drop caused by internal resistance of the battery 59. In the controller 61, the voltage drop is also generated in the switching element or the like. Furthermore, the voltage drop is generated in a cable connecting the controller 61 and the motor 54. As a result, the voltage actually supplied to the motor 54 is decreased to about seven volts. Therefore, the torque of the motor 54 is decreased to result in a problem that the steering assist force is not sufficiently obtained.
In order to deal with the problem, in FIG. 11, the boosting circuit 60 is provided to boost the voltage of the battery 59. For example, the boosting circuit 60 includes an oscillation circuit which outputs a pulse, the switching element which performs a switching operation with the pulse, a coil which repeats accumulation/release of electric energy by the switching operation to generate the high voltage, and a capacitor which smoothes the boosted voltage. For example, Japanese Unexamined Patent Application No. 2005-51951 describes the detailed boosting circuit. The boosted voltage can be imparted to the motor 54 to obtain the large steering assist force by providing the boosting circuit 60.
However, in the conventional apparatus, because the boosting circuit 60 always supplies the boosted voltage to the motor 54, the switching element constantly repeats the on/off operation in the boosting circuit 60, which results in the problem that power loss is increased in the switching element. There is a possible method, in which a threshold is set to the command value (for example, PWM signal duty ratio), the voltage is not boosted by the boosting circuit until the command value exceeds the threshold, and the voltage is not boosted by the boosting circuit when the command value exceeds the threshold. Therefore, when the command value exceeds the threshold, namely, only when the boosting is required, the switching element performs the on/off operation to boost the voltage, so that the power loss can be suppressed in the switching element. For example, Japanese Unexamined Patent Application No. 2003-153584 and Japanese Unexamined Patent Application No. 2003-200845 describe such boosting control techniques.
In Japanese Unexamined Patent Application No. 2003-153584 and Japanese Unexamined Patent Application No. 2003-200845, the PWM signal duty ratio is computed based on the command value computed from the deviation between the target value and the detected value of the motor current. The voltage is not boosted when the computed duty ratio is not more than 100%. When the computed duty ratio exceeds 100%, the switching element is turned on and off at the duty ratio corresponding to the excess amount of computed duty ratio, which allows the boosted voltage to be supplied to the motor drive circuit according to the excess amount of computed duty ratio.